-LRB- CNN -RRB- -- As many as 100 homes could be affected by flood waters in Wisconsin due to the failure of a 120-year-old sand levee along the Wisconsin River .

The levee , near the city of Portage in Columbia County , began to give way Sunday night , according to the National Weather Service 's Milwaukee/Sullivan office .

Patrick Beghin , a representative of the Columbia County Emergency Operations Center , confirmed to CNN Monday that the levee had in fact failed .

`` Once the levee completely fails ... it is unknown how far south the flood waters of the Wisconsin River will travel , '' the Weather Service said Sunday night .

Portage Mayor Ken Jahn later told CNN that while the levee had n't completely broken yet , it had deteriorated . Local prison inmates were being used to help with sandbagging , he said .

The levee is located on the south side of the Wisconsin River , just south of Portage . The weather agency urged residents to move to higher ground . Roadways , including parts of Interstate 39 , could close .

Authorities in Portage worked to evacuate residents as the levee approached imminent failure after heavy rainfall soaked the Midwest last week .

An alert sent out by Columbia County Emergency Management -LRB- CCEM -RRB- on Sunday urged residents near Blackhawk Park to evacuate immediately ahead of the flooding , which is expected to wash out a main road leading to about 150 residences .

`` Emergency vehicles including police , fire and EMS will not be able to reach residents , '' the statement said .

Kathy Johnson , the deputy director of the county 's emergency management department , said Sunday it was unclear how many residents remained in the area .

`` Anyone in there right now wo n't be able to come out , '' she said .

A Red Cross shelter was opened at a nearby church to accommodate displaced residents .

The river at Portage is now expected to stay above flood stage -- 17 feet -- through Wednesday , Beghin said . Portage wo n't be considered to be out of danger until the river has dropped below flood levels , he added .

Beghin noted that the flooding situation is not as widespread as it was during a similar 1993 flood , even though the river has now crested at a higher level than it did 17 years ago .

The Wisconsin River crested around 12 a.m. Monday at 20.56 feet -- roughly 3.5 feet above flood level , according to Beghin and the National Weather Service , putting major pressure on the Caledonia-Lewiston levee system .

Forecasts suggest the river at Portage will undergo a second peak in height on Tuesday morning , adding more pressure to the compromised levee .

Officials with the Department of Natural Resources have been monitoring the failed levee for days and trying to repair problem areas . But the levee system , built in the 1890s , was constructed from locally available materials -- mostly sand -- `` without any engineering design or adherence to any standards , '' the Natural Resources department said in a statement last week .

`` This is a 120-year-old relic , '' Wisconsin Department of Natural Resources spokesman Greg Matthews said Sunday . `` It 's made of sand . ... A modern levee that our engineers would be familiar with would be constructed of steel and concrete . ''

The levee 's failure comes after a week of rainfall that dumped as much as 11 inches of rain in parts of Wisconsin and neighboring Minnesota . Portage itself , however , did not receive any of the heavy rain .

Floodgates along the Menominee River in Niagara , Wisconsin , were opened in recent days to ease pressure on dams . Video showed a torrent of water gushing from an overflowing dam .

`` I have never seen the water rage down like it is , '' CNN iReporter Jason Asselin said .

CNN meteorologist Jacqui Jeras said Sunday that there 's no immediate end in sight to the flooding .

`` Even though some of these rivers have crested ... keep in mind that they 're still in flood , so this is going to be a problem for a couple of days , '' Jeras said . `` Many of these rivers are all going to be dumping into the Mississippi River which then in turn will rise up and we 'll see some flooding there , maybe in La Crosse and the Winona areas later in the week . ''

CNN 's Don Lemon and Rob Marciano contributed to this report .

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NEW : The Wisconsin River at Portage is expected to peak again Tuesday morning

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The levee on the Wisconsin River has now failed , according to a local emergency official

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The aging levee system is under pressure after heavy rainfall last week

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Flooding is expected to continue over the next few days